The present invention relates to non-caking sodium chloride (salt) compositions comprising iron, titanium and/or chromium complexes of hydroxypolycarboxylic compounds as an anti-caking additive, to a process to make such non-caking sodium chloride compositions, as well as to the use of such non-caking sodium chloride compositions to make brine, a solution of essentially NaCl in water, for electrolysis in, preferably, membrane cells.
GB 908,017 discloses the use of ammonium ferric salts of hydroxy-polycarboxylic compounds as anti-caking agents for salt. It is stated that, xe2x80x9cWhere the number of acidic, for example carboxylic, functions is in excess of the valence of the iron, the excess, in neutral compounds, is neutralised by a basic atom or molecule such as an alkali or alkaline earth metal atom or, preferably, an ammonium radical.xe2x80x9d It is not stated that the compounds must be neutral. Also, only ammonium ferric compounds are disclosed, while it is not shown that neutralising species other than ammonia can be used. Furthermore, it is disclosed neither that salt is not caking when it is treated with ferric compounds neutralised with a product other than ammonia, nor that it is advantageous to use other neutralizing agents than ammonia. In this respect, reference is made to British Chemical Engineering Vol. 11, No 1 (January 1966), pages 34 and 35, where numerous compounds were evaluated for their efficiency in rendering salt crystals non-caking. The majority of the evaluated compounds were shown not to be effective. The effective compounds all contained one or more nitrogen atoms or undesired heavy metals. Therefore, heavy metal-free anti-caking agents were considered to inevitably contain nitrogen. Typically, the nitrogen is present in the form of cyanide or substituted ammonia groups. Till the present day, sodium or potassium ferrocyanide has been the product of choice. However, the use of anti-caking agents containing nitrogen, especially in the form of cyanide groups, is undesired. More particularly, there is an ongoing debate in respect of the desirability of a sodium or potassium ferrocyanide in table salt. Furthermore, the use of sodium or potassium ferrocyanide, or other nitrogen-containing anti-caking agents, gives difficulties in electrolysis operations because of the formation of NCl3. Especially when the NCl3 accumulates, which is the case if chlorine gas is liquified as commercial in electrolysis operations, its formation is highly undesired because the resulting product is explosive. FR 69.36254 proposes to use ferric acetate, which is said not to suffer from these disadvantages, as an anti-caking agent for salt. However, ferric acetate was found not to be a sufficiently effective anti-caking additive for salt.
A further disadvantage of commercially used potassium ferrocyanide is the fact that the iron introduced by this agent can only be removed from brine produced from salt containing said anti-caking agent if special decomposition unit is used. Especially when the brine is used in membrane electrolysis cells, the iron that is not removed will precipitate, typically in the form of the hydroxide, in and on the membrane. This leads to less efficient membrane electrolysis operations.
For these reasons, the search for improved anti-caking salt additives has been ongoing and the need for improved non-caking salt compositions still exists.
Surprisingly, we have now found that non-caking salt compositions can be produced which do not suffer from the above-mentioned disadvantages. These non-caking salt compositions, where the salt is predominantly sodium chloride, are characterized in that they
are essentially nitrogen-free,
comprise a metal complex of hydroxypolycarboxylic acids, with the molar ratio between metal and hydroxypolycarboxylic acid being from 0.2 to 10, wherein the metal is selected from iron, titanium and/or chromium,
and have a pH of 1-10, preferably 3 to 9.
The pH requirement may be met without further additives being used, depending on the pH of the salt that is used. If the requirement is not met, then optional pH control agents can be used to obtain the desired pH.
The term xe2x80x9cpredominantly sodium chloridexe2x80x9d is meant to denominate all salt of which more than 50% by weight consists of NaCl. Preferably, such salt contains more than 90% by weight of NaCl. More preferably, the salt contains more than 92% of NaCl, while a salt of more than 95% by weight NaCl is most preferred. Typically, the salt will contain about 2.5-3% water. The salt may be rock salt, solar salt, salt obtained by steam evaporation of water from brine, and the like.
The term xe2x80x9cessentially free of nitrogenxe2x80x9d is used for compositions that during electrolysis operation do not form NC3. Generally, this means that only traces of nitrogen-containing species (other than inert N2 gas) are allowed in the composition. The amount of nitrogen atoms of said species in the composition is preferably less than 1 mg/kg, more preferably less than 0.1 mg/kg, while an amount of less than 0.01 mg/kg is most preferred. Higher amounts of such nitrogen are highly undesired, since they make the salt less suitable for use in membrane electrolysis operations.
The preferred pH range of the salt composition, measured as described below, depends on the type of hydroxypolycarboxylic acid used. For instance, it was observed that for iron-citric acid complexes, the preferred pH range is 6-10, since at this pH the best anti-caking performance was observed. For iron-mesotartaric acid complexes, on the other hand, the preferred pH range is 2-9, more preferably 3-7, while a pH ranging from 4 to 5 is most preferred. Which exact pH range works best for the other hydroxypolycarboxylic acids can be established simply by evaluating the caking behaviour of salt that has been treated with metal complexes of these products at the various pHs. The pH can be adjusted, if so desired, by means of any conventional acid or base. The acid or base can be added separately or together with the anti-caking agent. For the final composition to be nitrogen-free, the acid and base cannot be selected from nitrogen-containing products. Preferably, the pH of untreated salt is first adjusted to the desired level, after which a solution comprising one or more of the metal complexes of hydroxypolycarboxylic acids with the same pH is added to the salt. The way the anti-caking agent and the acid or base are introduced depends on the desired water content of the resulting salt and the water content of the salt to be treated. Typically, a concentrated solution of the agents is sprayed onto the salt.
If so desired, an additional pH buffer may be added to the salt and/or the treatment solution. The buffers to be used are of the conventional type. Preferably, they are organic acids. More preferably, they are carboxylic acids. Most preferably they are carboxylic acids that do not contain xe2x80x94CH3 and/or xe2x80x94CH2xe2x80x94groups for the reason given below, such as formic acid and oxalic acid. The acid in the buffer of choice preferably has a pK value in aqueous solution around the desired pH, as is known in the art. The mesotartrate anti-caking agent was found to be best combined with formic acid as the pH buffer. The pH buffer can be used with or without the optional pH control agent being used. The pH buffer can be introduced into the salt composition by spraying the pure compound, a separate solution, and/or by introduction after mixing with the anti-caking treatment solution. Preferably, a treatment solution is sprayed onto the salt which comprises a metal source, hydroxypolycarboxylic acid, optionally a pH control agent, and optionally a pH buffer.
The metal source to be used to make the metal complexes of hydroxy-polycarboxylic acids according to the invention can be any conventional, water-soluble metal salt. Preferably, the salt is essentially nitrogen-free as in chlorides, sulfates, and the like. The metals that can be used are iron, titanium and/or chromium.
It was observed that the presence of other metals does not remove the beneficial non-caking effect of the metal complexes according to the invention. Therefore, it is not necessary to use 100% pure metal sources. They may be combined with other metals that are less active or inactive, or may be contaminated with metals that are less desired, such as aluminium. Preferably more than 1, more preferably more than 5, most preferably more than 10% by weight of all metal in the composition is selected from iron, titanium and/or chromium. If the total amount of metal in the formulation has to be kept to a minimum, it is preferred that more than 25, more preferably more than 50, even more preferably more than 75, and most preferably more than 90% by weight of all metal in the salt composition is selected from iron, titanium and/or chromium. For various reason, including the fact that iron can be removed easily from brine if it is not complexed too strongly, as in the present case, the use of iron complexes is most preferred.
The hydroxypolycarboxylic acids that can be used according to the invention are selected from compounds having from 3 to 10 carbon atoms, one or more hydroxy groups, and two or more carboxylic acid groups, or mixtures of such acids. Acids that can be used according to the invention include citric acid, tartaric acid, saccharinic acid, ascorbic acid, saccharic acid, mucic acid, and isomers thereof. Complexes of iron, titanium, and chromium with these hydroxypolycarboxylic acids were found to render salt non-caking at low concentrations. Preferably, the hydroxypolycarboxylic acids do not comprise xe2x80x94CH2xe2x80x94 and/or xe2x80x94CH3 groups, since the presence of such groups was found to result in the formation of undesired chloroform and/or other chlorinated organic compounds in electrolysis operations. Said chlorinated organics, i.e. chloroform, contaminate the chlorine that is produced from brine containing said acids. Examples of preferred hydroxypolycarboxylic acids are tartaric acid, mucic acid, and saccharic acid. The use of tartaric acid, particularly mesotartaric acid of formula 
has several advantages over the use of other hydroxypolycarboxylic acids because its use i) results in an excellent anti-caking effect at the indicated pH range, ii) gives a favourable strong dependency of anti-caking performance on pH, iii) allows the easy removal of iron from brine produced with salt comprising the iron complexes of this acid, and iv) because residual (meso)tartaric acid ions in brine solutions do not interrupt membrane electrolysis operations. The product when used in electrolysis operations did not result in the formation of NCl3, chloroform and/or otherchlorinated organic compounds. For this reason, and because mesotartaric acid was found to be the most effective anti-caking agent, mesotartaric acid is the most preferred hydroxypolycarboxylic acid. Because it was observed that mesotartaric can be used in combination with one or more other hydroxypolycarboxylic acids without a dramatic decrease in performance being observed, also such mixtures can be used. If a mixture of acids is used, it is preferred that at least 5, preferably more than 10, more preferably more than 20, even more preferably more than 35, and most preferably more than 50% by weight of all acid in the formulation is mesotartaric acid.
A preferred mixture of tartaric acid, which includes mesotartaric acid, can be prepared in a conventional way by treating a natural or synthetic tartaric acid (CAS registry numbers 87-69-4 and 147-71-7, respectively) solution with concentrated NaOH at temperatures above 100xc2x0 C. Part of the L-, D-, and/or DL-tartaric acid is then converted to the desired mesotartaric acid (CAS registry number 147-73-9). The use of the nitrogen-free metal complexes of hydroxypolycarboxylic acids as an anti-caking agent was also found to bring the additional benefit that water that adheres to the salt is less likely to segregate upon storage.
It is noted that because of the pH dependency of the anti-caking agent based on hydroxypolycarboxylic acids, and in particular (meso)tartaric acid, it is possible to form blocks of salt from non-caking salt merely by changing its pH to a value at which the anti-caking effect does not exist and subsequently applying pressure. Such blocks can be used, for instance, in salt dissolvers, e.g. in water softening installations, where such salt blocks show less bridging. However, they may also be used as salt licks for animals. Residual iron complexes of hydroxypoly-carboxylic acid in such salt licks are not considered to be a problem.
In membrane electrolysis operations, the use of (meso)tartaric acid based anti-F caking agents has the benefit that if (meso)tartaric acid enters the electrolysis cell, it does not harm the membrane (no deposit is formed) while it is rapidly decomposed in the anode chamber, releasing only harmless gaseous products (typically just CO2). This is in contrast with various other hydroxypolycarboxylic acids, such as the less desired citric acid which was found to generate chloroform. Furthermore, it was found that brine obtained by the dissolution of non-caking salt according to the invention can be purified, i.e. freed of iron, with greater ease than brine containing alkali iron cyanide complexes, probably because of the weaker complexing power of the hydroxypolycarboxylic acids. The improved removal of metal from the brine prolongs the life of the membranes in brine electrolysis cells, also in view of the fact that the voltage drop over the membrane remains more constant over time, because less metal, i.e. iron, is carried into the cell and, consequently, less metal hydroxide or oxide is deposited in and on the membrane. The fact that the metal, i.e. iron, is now removed more easily allows for substantial savings in the brine purification step and the electrolysis operation. The metal, i.e. iron, removal step can be performed in the conventional way by increasing the pH of the brine to precipitate the hydroxide and subsequent removal of the hydroxide by filtration. For these reasons, a preferred embodiment of the invention is a membrane electrolysis operation using brine obtained by the dissolution of a salt composition according to the invention. More preferably, such an operation includes a process step wherein metal ions are removed from the brine.
Because the valency of the metal in the salt may vary and because different types of hydroxypolycarboxylic acids, with various amounts of carboxylic acid groups per molecule, can be used according to the invention, the molar ratio of metal to hydroxypolycarboxylic acid may vary over a wide range. If iron is used as the metal, both di- and tri-valent ions (ferro- and ferri-ions, respectively) are used with success. Practically, the metal in the final salt formulation will be present in all valencies. Therefore, the term metal complex of hydroxypolycarboxylic acid is used throughout this specification to denote compositions comprising metal ions in various valencies and a hydroxypolycarboxylic acid moiety in ionic form. If iron is used as the metal, ferro-compounds are preferred, because they were found to give a slightly better anti-caking performance.
The amount of hydroxypolycarboxylic acid in respect of the amount of metal ions will depend on the overall valency of the metal ions and on the nature of the hydroxypolycarboxylic acid, particularly the amount of carboxylic acid substituents per mole of acid. For the non-caking salt of the present invention, a suitable molar ratio between iron and hydroxypolycarboxylic acid is from 0.2 to 10. However, for the various hydroxypolycarboxylic acids different optimum ratios were found by simply evaluating the caking behaviour of the salt to which the products were added. For citrates, for example, a preferred range for the molar ratio of iron to acid is 0.75 to 2. For the preferred (meso)tartaric acid salts the preferred range was found to be 1.5 to 3.
The metal complexes of hydroxypolycarboxylic acids are preferably used in an amount such that less than 20 mg of metal per kg is introduced into the final non-caking salt formulation. More preferably, the amount used introduces less than 10 mg metal per kg of the formulation, while most preferably, the amount of metal introduced is less than 5 mg/kg. A preferred non-caking composition according to the invention includes about 3 mg/kg of Fexe2x80x3 and 16 mg/kg of tartaric acid ions, most preferably mesotartaric acid ions.
The metal complexes of the hydroxypolycarboxylic acids can be introduced or formed in and on the sodium chloride in various conventional ways. However, a preferred way that resulted in much better control of the anticaking performance was to dissolve the metal source, the hydroxypolycarboxylic acid, and optional further components in brine. To this end, one or more metal sources and one or more hydroxypolycarboxylic acids are introduced into a solution of salt (NaCl), optionally after the pH of said solution has been adjusted and/or buffered, with a salt concentration from 10% by weight (% w/w) to saturated. More preferably, the salt concentration in this solution is from 15 to 25% w/w. Most preferably, the salt concentration is about 20% w/w in said solution. Preferably, the metal and the hydroxypolycarboxylic acid(s) are provided on the salt crystals in a conventional way by spraying a solution (preferably in brine) onto the salt. In a preferred embodiment, the solution sprayed onto the salt comprises 20% w/w of salt, an iron source, such as FeCl2, in an amount that will result in about 5 g/kg of Fexe2x80x3 in said solution, and about 25 g/kg of (mesotartaric acid ions. If so desired, the salt is dried further after the addition of the iron complexes of hydroxypolycarboxylic acids or solutions thereof.